Method and device for producing brush products

ABSTRACT

A multi-stage injection molding method for the brush body is proposed in the first injection stage of which sealing of the bores of the bottom plate of the injection molding tool, including the tips of the cords of bristles that protrude into the mold interior is performed, and with the opening of the injection molding tool, a zone of rigidly fastened cords of bristles is created between the brush body, fastened in the intermediate plate, and the bottom plate, so that the cords of bristles can subsequently be served in the open without a counterpart blade, using a severing knife that swings back and forth, without the need for remachining the cut faces.

BACKGROUND OF THE INVENTION

The invention relates to a method and a device for producing brushproducts, such as brushes, brooms, artists' brushes and the like, usingendless cords of bristles, whose brush bodies are injection molded fromplastic, and the bristle sheaves are simultaneously cast integrally inthe form of tips of the endless cords of bristles introduced into themold interior through bores in the bottom plate.

Since the introduction of plastic bristles in the manufacture of brushproducts, technological solutions for producing these products usingendless cords of bristles have been sought.

The so-called “anchor method”, which is to be the point of departure asan example below and that continues as before to be employedpredominantly in brush production, in which bristle sheaves are stuffedinto prefabricated/predrilled brush bodies with the aid of a bristlestuffing machine and are secured to the bottom of the hole with wire ortiny plate anchors, has many disadvantages.

Not only the very high equipment costs but in particular also questionsof quality of the finished products have led to the search foralternative solutions. In mechanically stuffing the bristle sheaves intoprepared bores of the brush bodies, for instance, tiny interstices andgaps between the walls of the drilled holes and the filaments of thebristle sheaves, as well as between the filaments of the bristle sheavesthemselves, are unavoidable. These interstices and gaps are then favoredsites for mildew and the like and promote soiling.

To solve the technological and quality problems, various fundamentallydifferent courses have been taken, such as connecting the ends of thebristles and the corresponding bores in the brush body by fusing ormelting, or, in a particularly rational solution, directly embedding theends of the bristles in the plastic brush body at the same time thebrush body is made in an injection mold.

It has also been proposed that the tips of bristle cords be passedthrough bores in the bottom plates of an injection molding tool into themold interior for the brush body and jointly sheathed in this brush bodyin a single operation in the injection molding.

The most difficult, and until now practically unsolved, problem inembedding the bristle ends in the course of the injection molding of thebrush body is the reliable avoidance of overshooting as a result of thebores in the bottom plate of the injection mold that are needed forintroducing the cords of bristles into the mold interior.

By a known method disclosed in East German Patent Disclosure DD 293 718A5, bristle sheaves of endless bristle cords of plastic are used forbrush goods, in that the tips of the cords are passed through bores inthe bottom plates of the injection molding tool into the mold interiorsand simultaneously jointly injected into the brush body without furthermachining in the injection molding of the brush body.

These method proposals state that overshooting of the composition couldbe avoided with the aid of conical bores, tapering toward the moldinterior, in the bottom plate that terminate cylindrically, allowing aslight pressure of the cords of bristles to be attained. Many practicaltests, however, have shown that this cannot effectively, in terms ofequipment and/or process technology, counteract the tendency of theinjection molding composition, as a function of the injection pressureand the degree of the temperature-dependent viscosity, to escape to theoutside during the injection molding process between the filaments ofthe cords of bristles and between the cords of bristles and the walls ofthe bores. Furthermore, even the slightest escapes of the composition inthe case of conically shaped bores leads to the immediate formation ofplastic plugs in these bores, which quite effectively prevent the cordsof bristles from being pulled through the bores in the bottom plate inthe final molding, and as a rule this causes interruptions in productionwith sometimes severe underinjections on the bottom plates of theinjection molding tools. With the provisions proposed in these methods,the problems of reliable avoidance of overshooting during the injectionmolding process cannot be solved.

One method for producing brush bodies with a bristle field fortoothbrushes, the brush bodies being formed in some regions of at leasttwo plastic components, is known from German Patent Disclosure DE 44 39431 A1. In this method, bristle sheaves already cut to the proper lengthand assembled into bristle fields and located in a bristle mount areinjected jointly with the production of the brush body in a firstoperation. In other words, the use of endless cords of bristles is notthe point of departure. Nor does this reference show how the bristlesheaves are introduced with frictional engagement into the bristlemount, whose assembly is supposed to be done outside the tool inseparate operations. In particular, it does not show how the intrusionof injection molding composition into the bores of the bristle mount,which could occur despite the frictionally engaged packing of thebristle sheaves, should be prevented. For the reasons given, this methodis again unable to solve the problems described.

A further major problem in producing brush products using endless cordsof bristles is severing the cords of bristles after the conclusion ofthe injection molding process in conjunction with furnishing therequisite bristle lengths for the next injection molding operation,inside an automated, continuous production sequence.

In DD 293 718 A5, the severing of the finished brush products from thecords of bristles that remain in the bores of the bottom plate of theinjection molding tool is done in the open. To that end, after the brushbodies have cooled down, the upper part and a framelike middle part ofthe injection molding tool is lifted, along with the hardened brushbody, from the bottom plate and in the process the cords of bristles arepulled to the required lengths through the bores in the bottom plate.The brush body is lifted from the bottom plate with the aid of anintermediate plate. Further details on the manner and function of thesevering device and on the creation of further required cuttingconditions for severing the bristle cords in the open without asupporting cutting edge are not disclosed in this proposal, however.

So far, attempts to sever the cords of bristles under the aforementionedpreconditions, whether with rotating knives or reciprocating knives,without a supporting cutting edge have failed. Many filaments of thecords of bristles to be severed are pulled through the bore of thebottom plate during the severing process, resulting in unstable cuttingconditions. The method presented cannot solve the existing problems, forthe reasons given.

From German Patent DE-PS 845 933, the severing of toothbrushes with theinjection molding tool open, using a rotating severing disk, is known;during the severing process, the toothbrushes are not firmly heldstably, for instance by means of an intermediate plate or some otherholding device. Severing the finished brush products from the bristlefields without establishing a stable position of the brush body, asproposed in this reference, is extremely difficult because of the actualevents that occur in such severing processes. The lack of a stableposition of the brush bodies during the severing process makes thepossibility of a severing cut over the entire bristle field highlydoubtful. Furthermore, during the severing operation, the brush bodiescan be torn by force out of their positioning and tumbled about in thetool, making stable production sequences in fact impossible.

Serving by means of a rotating severing disk is problematic for tworeasons in particular, without addressing the aforementionedinadequacies at all. First, a cut in the same direction results inpersistent forces of friction and tension in its direction of motion.These forces are transmitted by the rotary motion of the severing diskto the cords of bristles to be severed and to their individualfilaments, creating relatively strong tensile forces and forces tendingto rip the bristles out. As a rule, this causes bristles and bristlesheaves to be torn out of the brush body and causes cords of bristles tobe pulled out of the bores of the bottom plate of the injection moldingtool through which the bristle cords of the bristle field have beenpassed.

On the other hand, the high rotary speed of the severing disk, with anincreasing penetration depth, generates higher friction with an increasein the resultant heat of friction. This causes melting at the interfacesof the cords of bristles and sticking on the severing disk. Tests haveshown that it is hardly possible to avoid these phenomena, even byundercutting the severing disk. Once again, for the reasons given, thismethod cannot solve the existing problems.

SUMMARY OF THE INVENTION

The object of the invention is to develop a method and a device forproducing brush products of the type defined at the outset that withoutadditional provisions reliably avoids overshooting in the region of thebristle sheaves and assures stable securing of the bristles in the brushbody and after the conclusion of the injection molding process,furnishing the requisite bristle lengths for the next injection moldingprocess, assures severing of the bristle field of the finished injectionmolded brush product in the open without requiring remachining.

The method of the invention assures that the demands made of ahigh-quality brush product, such as a smooth, flat bottom face andintimate sheathing of the injected bristle ends in a way that ismaterially bonded as tightly as possible, thereby stably fastening thebristles in the brush body, are met. A prerequisite for attaining thestated object and reaching the aforementioned goal is the lowestpossible viscosity of the injection molding composition and the highestpossible pressure, in order to attain the highest possible injectionspeed and to avoid heat losses. According to the invention, this isattained in that in the first stage of the multistage injection moldingprocess that accomplishes the sealing, plastic composition is introducedinto the mold interior at a speed of over 100 mm/s and a pressure ofover 1.5·10⁸ Pa.

The avoidance of overshooting, despite the high injection pressure andthe high injection speed, is attained according to the invention in thatin the first stage, only just enough plastic composition so that thebottom plate, including the entire bristle field is covered with it, butwithout filling the mold interior completely.

With these parameters, and expediently in terms of equipmentsupplemented with a heating conduit as well, undesired cooling down ofthe injection molding composition in the first injection stage, untilthe composition arrives at the bristle field, is averted without anysubstantial head pressure being able to build up in the mold interior.

The high injection pressure in the first stage is thus purelytheoretical in nature and corresponds only to the high injection speedbut does not lead to an undesired build up of pressure in the moldinterior.

If this first injection of composition meets the cooled or temperedbottom plate in the mold interior, then as a result of the immediatetemperature loss, a thin layer of composition that directly touches thebottom plate leads to sealing of the perforated bottom plate. Thissealing is propagated, with a slight time lag, to the bristle cordswhere they emerge from the bores of the bottom plate.

This layer that brings about the sealing, which initially is very thin,especially in the region of the cords of bristles, and accordingly isalso very vulnerable, can, however, be utilized up to a certain pressurerange as an adequate barrier, at the transitions of the cords ofbristles to the bores of the bottom plate, to the incoming injectionmolding composition in the subsequent injection stages.

It is significant according to the invention that in the first stage,only as much composition as is needed to seal the bottom plate and toreach the requirements in terms of the quality of adhesion and locationof the bristle sheaves is injected. As a result, on the hand a heatbuildup on the bottom plate from incoming injection molding compositioncan be kept within limits, and on the other, an increasing sealingeffect at the surface of the bottom plate from the heat energydissipated from the layer of composition near the bottom can be attainedfaster.

In order to make maximum use of the increasing sealing effect, thecourse of injection has been prolonged chronologically, so that inaccordance with the tests performed, the final injection stage in whichthe mold interior of the tool is filled has been placed in a range ofabout 2.5 to 9 seconds after the onset of injection.

The buildup of a head pressure even before the mold interior of theinjection molding tool is completely filled by incoming injectionmolding composition entering at high speed, is made more difficult orprevented according to the invention by a reduction of the injectionpressure to values below 4·10⁷ Pa, in combination with a reduction inthe injection speed. In order to utilize the plastic behavior of theperipheral zones of the filaments of plastic bristle material at hightemperatures for a connection between the brush body and the cords ofbristles that is in part created not only by positive engagement butalso by material engagement, according to the invention the temperatureof the injection molding composition for the brush body is selected inranges that are approximately in very high ranges and above the flowtemperature (FT) or the crystallite melting temperature (KT) (pertainingto crystalline or partly crystalline polymers) of the bristle material.

For instance, bristle material of polyamide 6 (PA 6), which has a highKT of 265° C., is combined for the brush body with polypropylene (PP),which has a KT of only 165° C. With an injection temperature of the PPof 10° C. above the KT of the PA 6, good results have been attained withrespect to the quality of the end product, and overshooting has beenprecluded, even though in these temperature ranges, polypropylene has anextremely low viscosity.

Thus because of the recrystallization of the polyamide 6, it isadditionally attained that the bristle sheaves, after unmolding of thebrush body, stand rigidly upright and tightly bundled, in exactagreement with their position in the bores of the bottom plate duringthe injection molding process. Furthermore, by utilizing the shrinkageparameters (up to 2% for polypropylene), a further adhesion effect ofthe bristle sheaves in the brush body is attained.

As experiments have shown, with this injection molding technique, thetendency to overshooting in the region of the cords of bristles has beensuccessfully prevented in a simple, without extensive additional effortand expense for technology. Special pressings of the bristle sheaves inthe bores of the bottom plate are not needed either to achieve thiseffect.

After the conclusion of an injection molding process and after reachingthe opening position of the injection molding tool, the bristle field isclamped again immediately by the double plates of the bottom plates; theintermediate plate firmly holds the brush bodies stably counter to thetraction direction of the cords of bristles, and as a result a zone ofrigidly tensed cords of bristles is created between the brush body andthe bottom plate, and in this zone the cutting operation is effectedutilizing only the tension of the cords of bristles and the internalrigidity of the bristle filaments. The cord of bristles tensed rigidlyin this way is severed, taking into account the requisite bristle lengthfor the ensuing injection operation, with the aid of a severing knifethat swings back and forth, without any counterpart blade. By using thebottom plate embodied as a double plate, the clamping of the bristlefield and the unclamping are attained without problems for the entirebristle field with the technologically required scope.

By using a reciprocating flat severing knife, the buildup of persistentpressure and tearing out forces that act on the bristle clusters in onedirection is avoided. The development of the heat of friction when thebristle fields are severed is also greatly reduced, compared to asevering disk operating at a high rate of revolution, because of thesubstantially lower cutting speed of the flat severing knife, andbecause of the low frictional area of this knife, and thus the severingprocess is practically unimpaired.

To allow the intermediate plate to hold the brush body firmly andstably, its contours should be designed with the aid of bevels and/orsteps in such a way that when the injection molding tool is opened, thebrush bodies are lifted by this plate to the predetermined height yetcan nevertheless be ejected in the direction of the cover plate afterthe bristle field is severed.

This has the advantage that on the one hand because of the wideengagement areas of the intermediate plate, relatively high tensileforces for lifting from the bottom plate can be transmitted to thecontours of the brush body substantially more gently to the cooled brushbody than if ejector mandrels are used. Thus the cords of bristles ofthe bristle field are pulled through the bores in the bottom plate,which are opened for this time, without impairing the surface of thebrush body. On the other hand, as a result the severed brush productscan be ejected once the bristle field has been severed.

An advantageous embodiment is defined by claim 9. In the teaching ofclaim 9, the cover plate of the tool is moved upward in the openingmotion, past the intermediate plate, far enough that after the severingoperation, the finished brushes are ejected from the intermediate plateby the ejector mandrel without difficulty, and without leaving notchesbehind in the direction of the cover plate.

In the further embodiment of the invention, it is provided that therequisite preconditions for the successful use of the severing device inthe severing process are created, which serve to establish more stablecutting conditions for the flat severing knife moving back and forthcrosswise to the cutting direction.

To that end, it is necessary to determine the most favorable spacing ofthe sectional plane of the severing knife from the bottom plate. This isachieved by placing the sectional plane of the severing knife at adistance from the bottom plate that while overcoming the relatively highcutting resistance that is presented to the severing knife by the cordsof bristles still allows utilizing the internal rigidity of the cords ofbristles. This distance is defined by the internal rigidity of thebristle material, the diameter of the filaments, the thickness of thebristle cords, and the stable guidance of the cords of bristles in thebores of the bottom plate and is ascertained empirically. Findings thusfar show that this distance is between 2 and 3 times the diameter of thecords of bristles.

A further precondition for the successful severing process is masteringthe problem that the individual filaments of the cords of bristles,depending on their position, that is, whether they are located on theperiphery or in middle positions of the cords of bristles, are heldvariably firmly in the bores of the bottom plate.

Depending on their position inside the cords of bristles, the filamentsreact differently to the tensile forces occurring in the cuttingoperation. This behavior has a definitive influence on the design of thecutting process.

When the cords of bristles are clamped in the bottom plate, thefilaments located on the outside of the bore in the clamping directionare fixed substantially more strongly in their position than thefilaments located crosswise to the clamping direction or those locatedin the interior of the bores and of the cords of bristles. This is alsoclearly evidenced by the scars on these filaments that occur at theclamping points.

According to the invention, the prerequisite for the ability of thesefilaments to withstand the extreme tensile stresses at the moment oftheir least resistance is created by providing that the clampingdirection of the bottom plate agrees with the direction of thereciprocal motion of the severing knife. As a result, when the severingknife is used in the course of the alternating reciprocating motions,the filaments located in the most strongly fixed outer positions in thecords of bristles are located on the front and rear side, respectively,of the cord of bristles and thus are capable in the alternating motionsof presenting the greatest resistance to the tensile stresses of theknife.

For severing the cords of bristles, the invention uses a severing deviceknown per se, which has an electric motor, a reducing gear, and aneccentric element, with which a reciprocating flat severing knife isdriven. Once the tool is opened, the severing device is loweredcentrally in the injection molding tool between the bristle fields andguide paths crosswise to the direction of motion of the severing knife.

The advantage of the version according to the invention is above allthat with a single operation, on so the one hand finished brush productscan leave the injection molding tool, while at the same time bristlefields fully prepared for the next injection molding operation protrudethrough the bores of the bottom plate to the requisite length into themold interiors of the injection molding tool once the mold is closed,without requiring additional technologically complicated steps forfollow-up of the bristle cords.

In the embodiment proposed, it is for instance possible for bristlematerials of metal to be injected into the brush body as well. In thecase of steel wire, of the kind used to make hand wire brushes and inwhich no pressing actions whatever are possible, the invention has madeit possible to effectively prevent overshooting. Perfect adhesion of thewire sheaves that form the bristles in the brush body has been achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in further detail below in terms of anexemplary embodiment. In the associated drawings,

FIG. 1 shows a side view of an open injection molding tool, comprising atop part, bottom part and intermediate plate, with an integratedsevering device;

FIG. 2 is a plan view on the bottom part of the injection molding tool,which is equipped with two modules for producing handwashing brushes;

FIG. 3 shows the section A—A through the double plate of the bottom partin the position for clamping the cords of bristles; and

FIG. 4 shows the closed injection molding tool, with the material of thebrush body introduced in the first injection stage for sealing the boresof the bottom plate of the injection mold.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following example, as a plastic composition for producing handwashing brushes, polypropylene is used for the brush body and polyamide6 is used for the bristle material.

The injection molding tool schematically shown in FIG. 1 is built intoan injection molding machine in a known way and substantially comprisesa base plate 5, intermediate plate 11, cover plate 12, and severingdevice 4, with a severing knife 10, the eccentric element 15, and areducing gear.

The intermediate plate 11, which is integrated with the injectionmolding tool and with its internal contour forms the outer contour ofthe brush body 14, and the base plate 5 that receives the split bottomplates 8; 9 are disposed to be movable toward the cover plate 12.

The endless cords of bristles 3 are passed through the perforated bottomplates 8; 9 (FIG. 3), which comprise two plates that can be displacedrelative to one another, and except for the phase of tool opening areclamped and protrude into the mold interior 1 (FIG. 2) to a length withwhich they are to be injected into the brush body 14.

In a first injection stage, with an injection pressure of approximately1.5·10⁸ Pa and an injection speed of 100 mm/s, a volume of plasticcomposition such that the plastic composition sheathes the perforatedsurface of the bottom plate 9, including the tips of the cords ofbristles 2 that protrude into the mold interior 1, with plasticcomposition 13 (FIG. 4) but without completely filling the mold interior1, is injected through the heating conduit 6 into the mold interior 1,this volume for instance being about 60% of the volume of the brush body14.

In the ensuing stages, while avoiding the buildup of a head pressure inthe mold interior 1, the injection speed is reduced, for instance tovalues of 30 mm/s, 10 mm/s, 4 mm/s, down to 2 mm/s in the last stage.

Once the injection molded brush body 14 has cooled down, the clamping ofthe cords of bristles 3 by the split bottom plate 8; 9 is undone, andwhile the injection molding tool is being opened, the brush body 14 islifted gently by the intermediate plate 11 from the bottom plate 9 asfar as is required for the bristle length for the finished bristle fieldand the bristle length for the bristle field to be injected into thenext brush body.

In this opening operation, the cords of bristles 2 cast integrally intothe brush body 14 and now firmly anchored in it are pulled through thesplit bottom plate 8; 9 and clamped again in this bottom plate once thefully open position of the injection molding tool is reached.

Between the brush body 14 and the bottom plate 8; 9, this creates a zoneof rigidly tensed cords of bristles 2, which in the predeterminedsectional plane 7 are cut off in the open from the cords of bristles 2for the next product by the severing device 4, using a severing knife10, in a cut to be performed without a counterpart blade.

The appropriate spacing of the sectional plane 7 of the severing knife10 from the bottom plate 9 is determined by placing the sectional plane7 at a distance from the bottom plate 9 that still allows the internalrigidity of the cords of bristles 2 to be utilized for the severing cut.In the example shown, for a thickness of the PA 6 filaments of 0.3 mmand a diameter of the cords of bristles 3 of 2.2 mm, a sectional plane 7was fixed at a distance of 6.0 mm from the bottom plate 9.

The direction of motion of the bottom plates 8; 9, which have aninfluence on the degree of resistance of the filaments of the cords ofbristles 2 to the forces, originating in the severing knife 10, thatpull the bristle sheaves out of the injection molded brush body 14,matches the direction of the reciprocating motion of the severing knife10.

By means of the embodiment according to the invention, decisivetechnological difficulties that until now have in principle stood in theway of the use of endless cords of bristles of plastic in themanufacture of brush products using injection molding tools by directinjection of the bristle sheaves into the brush body are overcome.

REFERENCE NUMERALS

1 Mold interior

2 Cords of bristles

4 Severing device

5 Base plate

6 Heating conduit

7 Sectional plane

8 Bottom plate

9 Bottom plate

10 Severing knife

11 Intermediate plate

12 Cover plate

13 Plastic composition

14 Brush body

15 Eccentric element

What is claimed is:
 1. A method of producing brush products whose brushbodies are injection molded from plastic, comprising the steps of:providing an injection molding tool comprising a base plate thatsupports a pair of split bottom plates, the pair of split bottom plateseach including a plurality of bores and comprising an upper plate and alower plate which can be displaced relative to one another, theinjection molding tool further comprising an intermediate plate and thebase plate with the pair of split bottom plates cooperatively define aportion of a mold interior; providing a severing device comprising aflat knife that swings back and forth and which acts without a counterpart blade, the severing device being incorporated as a component of theinjection molding tool and being displaceable In a direction transverseto the back and forth swinging thereof; introducing a plurality ofendless cords of bristles into the bores of the bottom plates and intothe mold interior; injection, in a first injection stage, a plasticmaterial into the mold interior such that the bores of the bottom platesas well as the portion of the cords of bristles extending into the moldinterior are sealed, the first injection stage does not fulfill the moldinterior; in a plurality of further injection stages, filling the moldinterior with a plastic material, the further injection stages beingdifferentiated in terms of pressure, speed and volume; after theconclusion of the final injection stage, displacing the intermediateplate relative to the base plate and bottom plates to an openingposition after which the cords of bristles are clamped by the bottomplates, in the opening position the brush body is firmly held and thecords of bristles are rigidly tensed between the brush body and theupper one of the pair of split bottom plates; and wherein a cuttingoperation is subsequently performed by the severing device by displacingthe flat knife along a sectional plane between the brush body and theupper one of the split bottom plates and swinging the flat blade backand forth in a direction transverse to the direction of displacement,the cutting operation being aided by a tension of the cords of bristlesand an internal rigidity of the individual filaments thereof.
 2. Amethod as defined in claim 1, wherein in the first injection stage theplastic material is injected into the mold interior at a speed of over100 mm/s and a pressure of over 1.5×10⁸ Pa.
 3. A method as defined inclaim 2, wherein for the cords of bristles, plastics having a higherflow temperature or crystalline melting temperature compared to a flowtemperature or a crystalline melting temperature of the injectionplastic material for the brush body are used.
 4. A method as defined inclaim 1, wherein in the further injection stages an injection pressuredoes not exceed 4×10⁷ Pa.
 5. A method as defined in claim 1, whereinbetween the first injection stage and the conclusion of the finalinjection stage there is a period of time of at least 2.5 seconds.
 6. Amethod as defined in claim 1, wherein a temperature of the plasticmaterial for the brush body is in ranges that are approximately 10° C.above a flow temperature or a crystalline melting temperature of abristle material.
 7. A method as defined in claim 1, wherein metal wiresare used as a material of the bristles.
 8. A method as defined in claim1, wherein a cover plate of the injection molding tool is moved upwardsin an opening motion, passed the intermediate plate, far enough thatafter a severing operation, a finished brush product is ejected from theintermediate plate by an ejector without difficulty and without leavingnotches behind in a direction of the cover plate.
 9. A method as definedin claim 1, wherein the cutting of the cords of bristles by the knifetakes place at a spacing of the sectional plane from the bottom platesthat is defined by a material quality of the cords and bristles, athickness and number of filaments, and a length of fitting precision ofa guidance in the bores of the bottom plates of the injection moldingtool and amounts to approximately three times a diameter of the cords ofbristles.
 10. A device for producing brush products, comprising: aninjection molding tool comprising a base plate that supports a pair ofsplit bottom plates, the pair of split bottom plates each including aplurality of bores and comprising an upper plate and a lower plate whichcan be displaced relative to one another for clamping cords of bristlematerial, the injection molding tool further comprising an intermediateplate, the intermediate plate and the base plate with the pair of splitbottom plates cooperatively define a portion of a mold interior, theintermediate plate further being displaceable with respect to the baseplate and bottom plates to an opening position; and a flat severingknife incorporated as a component of the injection molding tool, thesevering knife being displaceable in a cutting direction for severingthe cords of bristle material when the injection molding tool is in theopening position, the cords of bristle material being severed along asectional plane between a formed brush body and the upper one of thepair of split bottom plates, the flat severing knife swinging back andforth in a direction transverse with respect to the direction of thedisplacement, the flat severing knife acting without a supportingcutting underlay and the cutting is further aided by a rigid tensionapplied to the cords of bristle material when the injection molding toolis in the opening position.